High density template: materials and processes for the application of conductive pastes

ABSTRACT

The present invention provides a method of ablative photodecomposition and forming metal pattern which attains high resolution, is convenient, and employs non-halogenated solvents. The present invention is directed to a process for forming a metal pattern, preferably circuitization on an organic substrate, preferably on a circuit board or component thereof, which comprises coating the substrate with an ablatively-removable coating comprising a polymer resin preferably an acrylate polymer resin and preferably an ultraviolet absorber. A pattern is formed in the polymer coating corresponding to the desired metal pattern by irradiating at least a portion of the polymer coating with a sufficient amount of ultraviolet radiation to thereby ablatively remove the irradiated portion of the polymer coating. Next the patterned substrate is coated with a conductive metal paste to define the metal pattern, and the conductive metal paste is cured. The remaining polymer coating is removed by solvent stripping with nonhalogenated solvents. The present invention further includes patterning electronic structures comprising multilayer circuitry using the above method. An excimer laser is used to form vias or through holes in the electronic structure while simultaneously patterning the polymer coating. This results in perfect alignment between the pattern formed in the polymer coating and the vias or through holes. High resolution circuitry is thus attainable when the electronic structure is subsequently metallized with a conductive metal paste.

Which is a divisional of application Ser. No. 08/118,010 filed on Sep.8, 1993, now U.S. Pat. No. 5,460,921.

BACKGROUND OF THE INVENTION

A method of forming metal patterns on a substrate, particularly formingthe patterned circuitry on high density electronic packaging, utilizes atechnique termed "ablative photodecomposition." This technique involvesdepositing an organic polymer coating on the substrate to be patterned.The organic coating, which is sensitive to radiation, is then patternedby irradiation from a laser, typically an excimer laser. Commerciallyavailable excimer lasers typically emit at either 193 nm, 248 nm, 308 nmor 351 nm. The lasers which emit at 193 nm and 248 nm both requirefluorine gas whereas the lasers that emit at 308 nm require a chlorinecontaining gas. Fluorine gas is more difficult to handle and dispose ofthan chlorine gas. In ablative photodecomposition, a large number ofphotons of a particular wavelength are directed to the coating in ashort time. The coating, which must be capable of absorbing at the laserwavelength, absorbs a significant portion of these photons and as aresult, many polymer chain fragments are produced in a small volume in avery short amount of time. This causes a localized increase in volumewhich cannot be sustained, and the pressure is relieved by the processof ablation, wherein fragmented chains explode and escape from thecoating, leaving an etched material. Thus, fine line patterns can beproduced by this technique, without the use of wet, or solvent etchingof the coating.

However, with existing ablative photodecomposition methods, one of thedisadvantages is that the patterned coating is typically stripped withhalogenated solvents; halogenated solvents are the subject of increasedenvironmental regulations. In addition, existing ablativephotodecomposition methods typically utilize electroless deposition ofmetal, typically copper, to fill the ablated pattern. Electrolessplating is time and space consuming and requires solvent baths whichprovide a source for the evaporation of solvents.

It would be desirable to have a convenient method of ablativephotodecomposition that utilizes nonhalogenated stripping solvents, thatdoes not require electroless plating and that provides high resolutionof metal patterns.

SUMMARY OF THE INVENTION

The present invention provides a method of ablative photodecompositionand forming metal pattern which attains high resolution, is convenient,and preferably employs nonhalogenated stripping solvents. In thepreferred embodiment, an excimer laser that emits at about 308 nm can beused, thereby eliminating the requirement of fluorine gas. The presentinvention is directed to a process for forming a metal pattern on asubstrate, which comprises coating the substrate with anablatively-removable coating comprising a polymer resin, preferably anacrylate polymer resin, and preferably an ultraviolet absorber. Apattern is formed in the polymer coating corresponding to the desiredmetal pattern by irradiating at least a portion of the polymer coatingwith a sufficient amount of ultraviolet radiation from an excimer laserto thereby ablatively remove the irradiated portion of the polymercoating, and if desired, to simultaneously ablate the substrate belowthe polymer coating to form vias or through holes. The patternedsubstrate is coated with a conductive metal paste to define the metalpattern. Then the conductive metal paste is cured and the remainingpolymer coating is removed by nonhalogenated stripping solvent. Thepresent invention further includes patterning electronic structurescomprising multilayer circuitry using the above method. An excimer laseris used to form vias or through holes in the electronic structure whilesimultaneously patterning the polymer coating. This results in perfectalignment between the pattern formed in the polymer coating and the viasor through holes. High resolution circuitry is thus attainable when theelectronic structure is subsequently metallized with a conductive metalpaste.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E schematically illustrate the process of ablativephotodecomposition according to the present invention.

FIG. 2 is a photograph of an electronic structure showing the highresolution attainable with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method of ablative photodecompositionand forming a high resolution metal pattern which is convenient, andpreferably employs nonhalogenated stripping solvents. The resolutionattainable is less than 1 micron. The present invention is directed to aprocess for forming a metal pattern, preferably circuitization on adielectric organic substrate, preferably on a circuit board or componentthereof, which comprises coating the substrate with anablatively-removable coating comprising a polymer resin preferably anacrylate polymer resin and preferably an ultraviolet absorber. A patternis formed in the polymer coating corresponding to the desired metalpattern by irradiating at least a portion of the polymer coating with asufficient amount of ultraviolet radiation to thereby ablatively removethe irradiated portion of the polymer coating. Next the patternedsubstrate is coated with a conductive metal paste to define the metalpattern, and the conductive metal paste is cured. The remaining polymercoating is removed by non halogenated stripping solvents.

The present invention further includes patterning electronic structurescomprising multilayer circuitry using the above method. A commerciallyavailable excimer laser is used to form vias or through holes in theelectronic structure while simultaneously patterning the polymercoating. This results in perfect alignment between the pattern formed inthe polymer coating and the vias or through holes. High resolutioncircuitry is thus attainable when the electronic structure issubsequently metallized with a conductive metal paste.

The process of the present invention will be described by reference toFIGS. 1A to 1E. Referring to FIG. 1A, electronic structure 10 comprisesdielectric layer 11, first conductive layer 12 on top of layer 11, andsecond dielectric layer 13 on top of conductive layer 12. Dielectriclayers 11 and 13 preferably comprise an organic polymer, such aspolytetrafluoroethylene typically filled with an inorganic material suchas SiO₂. Conductive layer 12 typically comprises metals such as copper,gold, aluminum, silver, molybdenum or a mixture of metals. An ablativelyremovable polymeric composition is blanket coated, such as by rollcoating, on top of dielectric layer 13, to provide polymeric coating 14.Preferably the polymeric coating 14 is a thickness of about 0.3 to 1.0mils, more preferably about 0.8 mils.

Preferably, then the electronic structure 10 is heated to typicallybetween about 70° C. and 150° C. for about 15 to about 30 minutes, toremove the bulk of solvent from the polymer coating 14. Then theelectronic structure 10 is baked at a final temperature of about 200° C.to about 250° C., preferably about 230° C. for at least about 90minutes, preferably 150 minutes to cure coating 14. The result is asmooth, hard and uniform polymer coating 14 on dielectric layer 13. Thepolymeric composition may also be coated on dielectric layer 11 to formpatterns on the outside surface of dielectric layer 11. Referring toFIG. 1B, the laser is aligned to the electronic structure by an x-ytable. Alternatively the laser may be aligned through a mask.

Polymer coating 14 is then excimer laser ablated into the desiredelectrical pattern, utilizing a fluence of 1-20 J/cm², preferably lessthan 12 J/cm². Preferably the excimer laser emits a wavelength of about308 nm; such excimer lasers have the advantage of utilizing xenon andchlorine containing gas as opposed to excimer lasers which emit at 193and 248 nm and which require fluorine gas. The excimer laser is alsoused to ablate underlying dielectric layer 13 and expose conductivelayer 12 at the same time polymer coating 14 is ablated, resulting invia 20 shown in FIG. 1. Through holes in electronic structure 10 canalso be formed at the same time polymer coating 14 is ablated. Thesimultaneous ablation of polymer coating 14 and dielectric layer 13results in perfect alignment between via 20 and the pattern formed inpolymer coating 14.

Referring to FIG. 1C, after the pattern, or mask is formed in polymercoating 14, conductive metal paste 21 is then applied, for example, byscreening using draw down techniques, into the defined regions includingthe vias and through holes. The conductive metal paste has bulkresistivity of about 50 to about 100 micro-ohms-cm, preferably about 65micro-ohms-cm, a contact resistance of about 0.1 to about 50micro-ohm-cm², preferably about 0.26 micro-ohm-cm², and is able towithstand temperatures of at least 350° C., preferably 380° C.Preferably the conductive metal paste 21 has the following properties:it is suitable to be applied by screening; it is easily removed from thesurface of the polymer coating; it has metal particle size less thanabout 5 microns; and it has paste viscosity of about 20,000 to about40,000, preferably about 30,000 cps. Suitable conductive metal pastescomprise metal powder, such as gold, silver and copper and a polymericvehicle comprising a polymer and solvent. Conductive metal paste 21 iscommercially available such as Staystik, 301 from Staystik Inc., orAblestik from Staystik Inc.

The substrate in which the conductive metal paste 21 is to be applied ispreferably placed on a flat vacuum screening plate. The conductive metalpaste 21 is selectively applied to the substrate, as identified by theablated polymer composition template, by conventional techniques such asutilizing a rubber squeegee that applies a force of about 20-30 psi at arate of about 0.5" /second. Referring to FIG. 1D, the electronicstructure is then baked at from about 125° C. to 220° C., preferably125° C. from 60 to about 120 minutes, preferably about 90 minutes. Thebake is in a conventional oven such as in a convection oven to permitthe metal paste to adhere sufficiently to the substrate and to removethe solvent vehicle from the metal paste, leaving primarily conductivemetal 22 in the voids of polymer coating 14 and via 20.

Referring to FIG. 1E, polymer coating 14 is then removed from theelectronic structure with a suitable stripping solvent. Suitablestripping solvents have a high boiling point, that is greater than about110° C., preferably greater than about 130° C., and are chosen so thatthey will selectively remove the polymer coating 14 without swelling anysurrounding organic dielectric materials. The preferred strippingsolvent is propylene carbonate. Preferably the stripping solvent isheated, preferably at about 150° C. other suitable stripping solventsinclude, for example, g-butyrolactone, N-methyl pyrrolidone, ethyllactate, propylene glycol methyl ether acetate, dimethyl formamide,diethylene glycol dimethyl ether. Preferably, the stripping solvent isnon-halogenated. Although halogenated solvents such as chloroform andmethylene chloride will strip the polymer composition, they are lesspreferred.

Following the removal of polymer coating 14, the electronic structure isrinsed with water.

The ablatively removable polymeric composition

The ablatively removable polymeric composition which when applied to thesubstrate provides polymer coating 14, preferably possesses thefollowing properties: chemical resistance, especially to the componentscomprising the conductive metal paste; toughness at the temperature ofuse, which is typically room temperature; good film formingcharacteristics, that is the polymer composition dries to a non-tacky,continuous film; suitability for screening; and thermal resistance,preferably to at least 70° C., more preferably to at least 240° C.Polymer coating 14 must be removable by the stripping solventparticularly after the conductive metal paste is baked.

The polymeric composition comprises a polymeric resin, or mixtures ofpolymer resins and optionally, though preferably, an ultraviolet (UV)absorber and a solvent. Where the polymeric composition is transparentat the wavelength of the excimer laser, such as PMMA at a wavelength of308 nm, an absorber is required. Preferably, the polymeric compositioncomprises by total composition, 5-20% by weight polymeric resin, 80-95%by weight solvent and 0-15% by weight of UV absorber. The polymericresin preferably comprises an acrylate polymer, such as, for example,polymethylmethacrylate, polyethylmethacrylate, polybutylmethacrylate,and mixtures or copolymers thereof. Polymethylmethacrylate is preferred,more preferred is polymethylmethacrylate (hereinafter PMMA) having anaverage molecular weight of between about 25,000 to about 250,000,preferably between about 125,000 to about 175,000. PMMA is commerciallyavailable from E.I duPont de Nemours under the trademark ELVACITE.ELVACITE 2021, 2008 and 2010 are preferred; ELVACITE 2041 is morepreferred. PMMA has the advantage of being both laser and mechanicallydrillable.

Suitable UV absorbers are those that absorb radiation at the wavelength-emitted by the excimer laser and will cause, through the transfer ofenergy, the ablation of the polymer coating upon exposure to UVradiation. Substituted hydroxyphenyl benzotriazoles, such as TINUVIN328, manufactured by Ciba-Geigy Corporation, is an example of a suitableUV absorber to be used in combination with a polymer resin, such as PMMAand a excimer laser that emits at 157 to 351 nm, preferably about 308nm. Other suitable UV absorbers include pyrene, coumarin and derivativesthereof.

When a UV absorber is included in the polymeric composition, then anorganic solvent is also used. The organic solvent provides uniformdispersion of the UV absorber, and preferably the organic solventpermits uniform and easy coating of the polymer composition. Examples ofsuitable organic solvents include diethylene glycol dimethyl ether,ethyl lactate toluene, acetone, methyl ethyl ketone, methylene chloride,ethyl acetate, tetrahydrofuran, acetonitrile and dimethyl formamide.

The polymer resin, and when employed, the UV absorber, are dissolved inthe solvent, and the polymer composition is coated onto dielectric layer13. It is to be understood that the polymeric coating 14, when applied,contains prepolymers, oligomers, or other materials polymerizable to apolymeric coating.

EXAMPLE 1

The polymeric composition was prepared by first heating 800 g ofdiethylene glycol dimethyl ether to 70° C., followed by the addition of88.9 g ELVACITE 2041 PMMA resin. After mechanically stirring for 3hours, the solution was cooled and 4.19 g TINUVIN 328 was added. Themixture was stirred for 30 minutes, resulting in a homogeneous polymercomposition comprising 89.6% solvent, 10.0% PMMA resin and 0.4% UVabsorber by weight.

The polymer composition was roll coated onto the top dielectric layer ofa substrate comprising a conductive layer of copper sandwiched betweentwo dielectric layers of SiO₂ filled polytetrafluoroethylene. Thesubstrate was then baked in a convection oven at 100° C. for 10 minutesto remove the solvent. The resulting coating was smooth and hard and hada thickness of 0.35 mil. The polymer coating and underlying dielectriclayer were excimer laser ablated at a wavelength of 308 nm, with a 25nanosecond pulse duration, into a pattern of aligned voids in thepolymer coating and vias. STAYSTIK 301, a conductive metal pastecontaining gold powder from STAYSTIK Inc., was screened into the voidsand vias using draw down techniques and baked at a final temperature of230° C. for 150 minutes to remove the solvent from the conductive metalpaste, and to coalesce the metal paste. The metal remained in the voidsand vias. The polymer coating was then stripped from the dielectriclayer with propylene carbonate at 70° C. in about 10 minutes. Thesubstrate was rinsed with a stream of water at 45° C. FIG. 2 shows thestructure of the resulting product.

EXAMPLE 2

The composition of Example 1 was prepared as in Example 1, except thatethyl ether was substituted for diethylene glycol dimethyl ether.

The invention is not limited to the embodiments of the electronicstructure or package and the method of producing a high density templatewhich have just been described; it is intended by the following claimsto include all technically equivalent means which come within the fullscope and true spirit of the invention.

We claim:
 1. An electronic structure having:a first dielectric layer; anablatively-removable, second dielectric layer comprising an organicpolymer; ablated, metallized, vias or through holes, disposed in thesecond dielectric layer; a conductive layer sandwiched between the firstand second dielectric layers; a layer of circuitization, composed ofcured conductive metal paste disposed on said second dielectric layer inelectrical and mechanical contact with said vias or through holes;wherein said vias or through holes in the second dielectric layer extendfrom the interface between the conductive layer and the seconddielectric layer, through the second dielectric layer, produced by theprocess comprising: (a) providing: a first dielectric layer; anablatively-removable, second dielectric layer comprising an organicpolymer; and a conductive layer sandwiched between the first and seconddielectric layers; (b) coating the second dielectric layer with anablatively-removable polymer coating comprising a polymer resin; (c)ablatively photodecomposing the polymer coating to form a desiredpattern in the polymer coating; (d) then ablatively photodecomposing thesecond dielectric layer to form vias or through holes in the seconddielectric layer extending from the interface between the conductivelayer and the second dielectric layer, through the second dielectriclayer, to the ablated portions of the polymer coating; (e) applying aconductive metal paste to the structure of step d, to definecircuitization in the patterned polymer coating and to metallize thevias or through holes in the second dielectric layer; (f) curing theconductive metal paste; and (g) removing the remaining polymer coatingwith a stripping solvent, to provide an electronic structure comprisingcircuitization composed of cured conductive metal paste, disposed on thesecond dielectric layer and in electrical and mechanical contact withthe cured conductive metal paste of the vias or through holes.
 2. Theelectronic structure of claim 1, wherein the electronic structure is acircuit board.
 3. The electronic structure of claim 2, wherein the firstand second dielectric layers comprise an organic polymer.
 4. Theelectronic structure of claim 3, wherein the organic polymer in one orboth of the dielectric layers comprises polytetrafluoroethylene.
 5. Theelectronic structure of claim 4, wherein the polytetrafluoroethylene isfilled with SiO₂.